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New V-FETs for my Sony TA-5650

S

seventies61

New Member
I found 4 working pairs of Sony SK60/SJ18 (rank 56) for my old non working Sony TA-5650. After replacing the four VD-1221 diodes with a series string of two 1N4148 diodes what must be checked (voltage etc...) to install my new transistors without burning them? Thanks.:tresbon:
 
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I'm not sure about this amp, haven't done a lot of work on FET-amp's.
It seems to me it can be fired up without the FET's installed to do some voltage-measuring to be sure the driving-circuit is OK.
Why only 4 FET's, there are 8 of them in total? Only one channel gone?
And I wouldn't fire up anything without first doing a recap. This amplifier is OLD and there couldn't be many electrolytic cap's left with their full capacity.
As I said, NOT a lot of experience with FET's so hopefully someone else will tick-in.

Kind regards,

/tri-comp

P.S.
Have the service-manual in case you need it. PM me with your e-mail address for a free pdf-copy.
 
Seems to me, back when these amps first came out, that the factory stressed that they be brought up on a variac to ensure they were safe to operate.

Tri-comp, do you have any of the service bulletins that Sony issued? I remember my shop back then had everything issued and recall _lots_ of updates for this series of amps. What I would give now for that stack of knowledge......

Cheers,

David
 
Nope, what I have is more than 500 Sony bulletins but they are more recent.
From late 80'ies and on.

I actually have ONE bulletin on the Sony FET's but I can't find it at the moment. I'll keep looking and I remember sending it to another forum.
I'll go there and look as well.

/tri-comp
 
dshoaf said:
Seems to me, back when these amps first came out, that the factory stressed that they be brought up on a variac to ensure they were safe to operate.

Tri-comp, do you have any of the service bulletins that Sony issued? I remember my shop back then had everything issued and recall _lots_ of updates for this series of amps. What I would give now for that stack of knowledge......

Cheers,

David
Click to expand...

Thought I had read and heard NEVER to use a Variac with VFET amps as it will simply destroy them... but I could be wrong...

Do a search here on AK on the TA-5650, there are whole threads covering all aspects of restoring these to working (safe) condition :)
 
Don't take any further action until someone with experience of fixing these units is able to contribute.
 
NEVER power up a VFET with any reduced-voltage supply...the VFET's are in full conduction mode by default, and require the 'pinch-off' voltage to prevent their destruction.

You may power up the amp with the VFET's removed with no problem, and I recommend you do this. Once powered up, check that the pinch-off voltages are present, and the high-current supply as well. The #61 connnector at the lower bottom right of the main amp board looks to be a convienent place to check these voltages. Pin 1 at the bottom should be the -44V high-current supply, pin 2, the next one up in line, should be the -76V pinch-off voltage. Pin 4 should be the +76V pinch-off, and pin 5 should be the +44V high-current supply.

If the +/-76V are missing or show any appreciable AC component, do not install the VFET's.
 

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Thank you! With V-FETS out, the #61 connnector at the lower bottom right of the main amp board gives these voltages: Pin 1 at the bottom : -50V high-current supply, pin 2, the next one up in line: -97V pinch-off voltage. Pin 4: +97V pinch-off, and pin 5: +50V high-current supply.

What about the readings on the source/gate of the V-FET conductor patterns on the board ?
 
What about 'em? You checked the important ones, so unless you have some burnt resistors or dead transistors, things are looking pretty good.
 
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